1. Field of the Invention
The present invention relates to an electromagnetic wave absorber molding material used for forming a molded element making up at least part of an electromagnetic wave absorber, a wave absorber molded element made of the molding material and a method of manufacturing the molded element, and to the electromagnetic wave absorber including the molded element.
2. Description of the Related Art
It has been commonly practiced to test electromagnetic compatibility (EMC) of a variety of types of instruments in anechoic chambers. The standards concerning EMC define permissible levels and testing methods to comply with for electromagnetic waves generated by the instruments. International standard organizations that establish the standards relating to the EMC include the TC77 and the CISPR that belong to the International Electrotechnical Commission (IEC). These organizations have made basic standards and common standards relating to the EMC.
The site attenuation measurement is used to evaluate the capability of anechoic chambers required for measuring and evaluating emission (of electromagnetic energy). According to the standard CISPR22 and the standard ANSI C63.4, the normalized site attenuation is used to define the conditions for the capability required for the anechoic chambers. To be specific, the required capability of the chambers is that the measured value of the site attenuation falls within the theoretical site attenuation value plus or minus 4 dB.
The electric field uniformity measurement is used to evaluate the capability of the anechoic chamber required for measuring and evaluating immunity (the capability of eliminating electromagnetic interference). It is common that the anechoic chamber has a metal floor since measurement and evaluation of emission are also performed in the chamber in many cases. An electromagnetic wave absorber is placed on the metal floor when immunity is measured and evaluated in such an anechoic chamber.
With regard to an immunity test of emission of electromagnetic waves having a frequency of 1 GHz or lower, the electric field uniformity required for the anechoic chamber is that, in 75 percent of grid-shaped 16 points arranged in a window of 1.5 meters by 1.5 meters, that is, in 12 points, the electric field intensity falls within the range of −0 dB to 6 dB of the test field intensity. A testing method for digital cellular phones has been recently added to the standard IEC61000-4-3 that requires the field uniformity for the anechoic chamber at frequencies up to 2 GHz.
A composite electromagnetic wave absorber made up of a combination of ferrite tiles and a dielectric loss material is one of wave absorbers used for the anechoic chamber intended for EMC testing. The composite wave absorber was first employed in the anechoic chamber of the Radio Research Laboratory of the Ministry of Posts and Telecommunications of Japan in 1969. This type of composite wave absorber is the dominating wave absorber used for the anechoic chamber intended for EMC testing.
The conventional composite wave absorber implements an electromagnetic wave absorbing capability in a wide band, taking advantage of features of each of the ferrite tiles and the dielectric loss material. The composite wave absorber is designed such that the ferrite tiles efficiently absorb waves in a low-frequency region around 30 MHz to 500 MHz and that the dielectric loss material efficiently absorb waves in a high-frequency region of 500 MHz and higher. The ferrite tiles are plate-shaped sintered ferrite. Ferrite used for the ferrite tiles is mainly Ni—Cu—Zn or Ni—Zn ferrite. The tiles have a thickness around 4 to 7 millimeters (mm). The dielectric loss material is mainly a, conductive material such as carbon mixed or soaked in a foam. The dielectric loss material utilized has a length around 45 to 250 centimeters (cm), the length being orthogonal to the ferrite tile surface.
A composite electromagnetic wave absorber made up of a combination of ferrite tiles and a dielectric loss material has been practically utilized as a wave absorber used for immunity testing in a wide range from a frequency as low as 26 MHz to a frequency higher than 1 GHz. A composite wave absorber made up of a combination of ferrite tiles and a magnetic loss material as disclosed in the Japanese Patent No. 3041295 has been recently brought to practical use, too. This wave absorber has a length around 10 cm, the length being orthogonal to the ferrite tile surface.
A non-flammable electromagnetic wave absorbing sheet and a wave absorbing structure made of the sheet are disclosed in Published Unexamined Japanese Patent Application 2002-176286.
It is preferred that wave absorbers used for the anechoic chamber is non-flammable. Non-flammable wave absorbers have been practically utilized for the anechoic chamber intended for EMC testing. The non-flammable wave absorbers incorporate a dielectric loss:material such as one made of a conductive material mixed in an inorganic material, or one made of a flat plate-shaped or honeycomb-shaped structure of an inorganic material to which a conductive material is applied or in which a conductive material is soaked.
However, this type of wave absorber incorporates the dielectric loss material so that it is required that the absorber have a length of at least around 45 cm. Therefore, the problem is that it is not preferred in some cases to place this wave absorber in a limited space such as a small acechoic chamber, in particular.
On the other hand, the composite wave absorber disclosed in the Japanese Patent No. 3041295 incorporates the magnetic loss material made up of an organic material in which a magnetic loss material is mixed or soaked. Therefore, this type of composite wave absorber does not exhibit excellent non-flammability and it is not suitable for use in the anechoic chamber for safety reasons.
The wave absorbing structure using the non-flammable wave absorbing sheet disclosed in Published Unexamined Japanese Patent Application 2002-176286 is honeycomb-shaped or corrugated. However, the wave absorbing structure using the non-flammable wave absorbing sheet has a problem that the manufacture of the structure requires a number of steps. For example, the honeycomb-shaped structure is fabricated as follows. First, the non-flammable wave absorbing sheets are formed. Next, a plurality of sheets are stacked while portions of adjacent ones of the sheets are bonded to each other to form a laminate. The laminate is then expanded.